This invention relates generally to the manufacture of a tensed mask cathode ray tube (CRT) and particularly to means for photoscreening the faceplate thereof through the tensed mask. Conventional color CRTs have a generally spherical shadow mask for shadowing a phosphor target or screen consisting of a regular pattern of photo-deposited triads of red, green and blue light-emitting phosphors from its corresponding electron-beam-emitting source located in the neck of the CRT. The foraminous shadow mask is used in forming the phosphor screen, and it must be repeatedly and precisely repositioned, with respect to the CRT faceplate, during screen forming. In most such CRTs, the shadow mask is supported on a relatively heavy frame that is capable of self-maintaining its configuration since the mask is not under tension. During fabrication of the screen, the shadow mask is repeatedly positioned in registration with the CRT faceplate by recourse to a plurality of stud-engaging flat springs that are attached to the shadow mask frame.
This invention concerns a tensed mask CRT. The tensed mask comprises a thin foil having a pattern of apertures. The mask is held under tension in position adjacent to a flat faceplate upon which the phosphor screen is formed. Since the foil mask is extremely thin, etching of the apertures can be precisely controlled, thus providing a CRT of much higher resolution. The foil is under significant tension, close to the yield point of the steel material generally used. It thus may be heated to a very high temperature before the tension is relieved and mask distortion sets in. The electron energy that a tensed foil mask can absorb without significant degradation in color purity is much greater than for a conventional spherical shadow mask tube. Consequently a much brighter and sharper image can be produced with a tensed mask CRT.
An inherent problem in a tensed mask CRT is that the mask support frame must be capable of withstanding the relatively high tensile forces exerted by the tensed shadow mask thereon without substantial deformation. One mask-supporting technique involves attaching the mask at its periphery to a glass or ceramic support frame by means of a devitrifiable glass or "frit". The mask is typically composed of a steel having a coefficient of thermal expansion that is significantly higher than that of glass. The mask is joined to its support frame under modest tension with a bead of devitrifiable frit in place and heated to the melting temperature of the frit. The steel mask expands at a much greater rate than the glass support frame. When the frit melts and the structure is subsequently cooled, the frit solidifies at a temperature at which the mask is still expanded, and firmly bonds the periphery of the mask to the glass support frame. As the solidifying temperature for the frit is sufficiently high, the steel mask is still enlarged relative to the support frame. Further cooling of the structure results in the mask being prevented from returning to its original size by virtue of its having been bonded to the support frame by the solidified frit. The mask is thus subjected to a large tension force. This high tension force is imparted to the glass support frame, and since the frame is not infinitely stiff, it deflects somewhat. This deflection is greater along the long sides of the frame than along the short sides. As a result the frame imparts a displacement of the aperture pattern from the position the pattern occupied before the frame cools down.
In accordance with conventional techniques for forming phosphor screens on the panels of color CRTs, the shadow mask is used as a stenciling element. It is therefore repeatedly positioned in accurate registration with the panel, and with respect to exposure light sources that simulate the electron beams emitted by the electron guns in the CRTs. It will be appreciated that any deformation in the mask support frame will establish the mask apertures in a distorted pattern. Thereafter, subsequent to the screening process, the phosphor screen formed on the panel will reflect a corresponding distortion.
In accordance with the referenced copending applications, the tensed mask support frame ultimately becomes a structural element of the completed CRT envelope. During a final manufacturing step, the CRT panel, support frame and funnel are subjected to an elevated temperature with devitrifiable frit being applied on corresponding mating surfaces. The frit devitrifies, and when it has solidified, it provides a unified glass structure that is hermetically sealed. Since the tensed mask is heated during the assembly process, it expands more than the support frame, and most of the tension in the mask is relaxed, thus substantially removing the deformation forces acting on the support frame. As the frit solidifies (during tube cool-down), the support frame is bonded between the faceplate and funnel of the CRT. On cooling of the tube, tension forces are again built up in the mask. However, these forces do not result in deformation of the support frame because the frame is now firmly bonded to the faceplate and funnel. Thus the aperture pattern in the mask of the finished CRT is slightly expanded and different from the aperture pattern in the mask during screening due to the tension-induced distortion of the mask's support frame during the screeing step. Thus color purity errors are apt to result. This invention is directed to elimination of the distortion of the support frame, and the attendant corresponding distortion of the mask aperture pattern, manifested during screening operations prior to the advent of the subject invention.
An early example of a tensed shadow mask for use in a color television cathode ray tube is described in U.S. Pat. No. 2,625,734. The tensed mask described therein was created by resort to a process called "hot-blocking". The practice was to insert a flat mask between a pair of frames which loosely received the mask. A series of tapped screws joining the two frames served to captivate the mask when the screws were subsequently drawn-down. The loosely assembled frame and mask was then subjected to a heat cycle by positioning heated platens adjacent the mask to heat and thereby expand it. The frame, however, was kept at room temperature. When the mask attained a desired expansion, the frame screws were tightened to captivate the mask in its expanded state. The heating platens were then removed. Upon cooling down to room temperature, the mask was maintained under tension by the frame. The resultant assembly was then mounted inside the tube adjacent the phosphor screen.
U.S. Pat. No. 4,069,567 describes a method for tensing a mask in which the mask is heated while the tension by passing an electrical current through it. Alternately, RF (radio frequency) heating is proposed. The metal of the mask has a greater coefficient of expansion than the structural frame that supports it, hence upon cooling, and upon captivation of the mask relative to the frame, the mask is held in a tensed state.
A method for processing color cathode ray tubes having a color selection electrode consisting of a thin metal foil with a pattern of electron-transmissive apertures formed therein is disclosed by Moore in U.S. Pat. No. 3,894,321, of common ownership herewith. The foil is stretched across the bulb of the tube and sealed directed thereto. According to this invention, such a tube may be exposed and screened without the necessity of resorting to a processing system which dictates the use of interchangeable masks. A lighthouse assembly for screening tubes having foil shadow masks is described and shown.